Measurement and Monitoring Methods in Ecohydrology III Posters
Presiding: B D Newman, Los Alamos National Laboratory; B Scanlon, Bureau of Economic Geology, University of Texas at Austin
H33B-01 1330h
Isotopic Fingerprint of a Hyporheic-Hypolentic Boundary
Kentucky Lake is located in western Kentucky (USA) and is the largest reservoir on the Tennessee River. Current management practices by the Tennessee Valley Authority (TVA) dictate that water level is raised 1.5 m in March and lowered 1.5 m over a three-month period beginning in August. Ledbetter Creek, a third-order tributary to Kentucky Lake, is located on the western side of the reservoir. The mouth of Ledbetter Creek spreads out across a mudflat before discharging to an embayment attached to Kentucky Lake. The mudflat is inundated following reservoir-stage increase in the spring, and is drained in the autumn. Stable isotopes of H2O and conservative solutes, such as chloride (Cl-), were used to define the hyporheic-hypolentic boundary at the mouth of Ledbetter Creek and trace water movement associated with reservoir-stage manipulation. Three water sources were defined in the Ledbetter Creek watershed, based on isotopic composition and Cl- concentration: ground water, stream water from Ledbetter Creek, and embayment water from Kentucky Lake. At winter pool, Δ2H and Δ18O values decreased across the hyporheic-hypolentic boundary from -36.8 to -42.5 per mil and -6.04 to 7.24 per mil, respectively. Cl- concentrations decreased across the boundary from 3.2 to 1.3 mg/l. The profile indicates that Ledbetter Creek infiltrated into the substrate to a depth of 10 cm near the confluence with the embayment. Below this depth, isotopic values and Cl- concentrations were indicative of ground water within the Ledbetter Creek watershed. Following reservoir-stage increase, Δ2H and Δ18O values shifted from -24.5 to -42.5 per mil and -4.87 to -7.46 per mil, respectively. Cl- concentrations shifted from 5.7 to 1.6 mg/l across the hyporheic-hypolentic boundary. At this time, surface water from the embayment infiltrated into the hyporheic-hypolentic zone to a depth of at least 16 cm below the channel bottom. Reservoir-stage decline in the autumn caused source-water mixing, largely obscuring the hyporheic-hypolentic boundary. However, based on these findings, stable isotopes provide a suitable alternative to conventional tracers for delineation of water masses within the hyporheic-hypolentic zone.
H33B-02 1330h
Response of Sap-Flow Measurements on Environmental Forcings
The exchange of water between the atmosphere and biosphere is an important determinant of climate and the productivity of vegetation. Both evaporation and transpiration involve substantial amounts of energy exchange at the interface of the biosphere and atmosphere. Knowing how transpiration changes throughout the seasonal and diurnal cycles can help increase the understanding of how a forest reacts to changes in the biosphere and atmosphere. A common way to estimate transpiration is by measuring the sap flowing through the living tissues of trees. A study was conducted at Morgan-Monroe State Forest, a mixed deciduous forest in south central Indiana (USA), to investigate how sap flow in trees responds to changes in meteorological and environmental conditions. The heat -dissipation technique was used to estimate sap velocities from two Big Tooth Aspen (Populus grandidentata) and two Tulip Poplars (Liriodendron tulipifera). Sap velocity patterns (normalized by a reference potential evapo-transpiration) were directly compared with meteorological and ecological measurements, such as vapor pressure deficits, photosynthetic active radiation (PAR), rain fall, and soil moisture content. In this study, we also investigated the uncertainties and problems that arise in using the heat dissipation technique to extrapolate the single-tree measurements to the forest scale.
H33B-03 1330h
Water Vapor Storage Change in the Canopy-Air Space of a Tall Deciduous Forest
The ability of weather and climate models to predict humidity, cloud formation and precipitation critically depends on the exchange of water vapor between vegetation and the atmosphere. The canopy air-space in tall forests is deep enough to act as a buffer volume that is depleted at times of well developed turbulent mixing, and gets recharged in conditions of poor mixing. Recent studies have attributed biases in modeled vapor exchange to the misrepresentation or neglect of this mechanism. At the Morgan-Monroe State Forest AmeriFlux site (Indiana, USA), water vapor exchange and the vapor storage change in the canopy air-space has been observed for the last six years. The objective of this work is to calculate vapor storage change fluxes in the canopy air-space from time increments of concentration profiles from data collected in 2003. We relate vapor storage change fluxes to measured environmental forcing quanitites, such as net radiation, ambient vapor pressure deficit, dew-point temperature depression, stability, and friction velocity to interpret the observed seasonal and daily patterns. Also, changes in water vapor storage rates are compared with measured latent heat fluxes to determine how the total forest-atmosphere vapor exchange is affected by the recharging and depletion of water vapor throughout the canopy air-space.
H33B-04 1330h
Technology for Evaluating Direct Recharge in a Juniper Dominated Karst Landscape
Non-native woody plant encroachment in semi-arid regions of the world has caused speculation about the influences of such encroachment on groundwater and surface water flows, interactions, and future availability. Our study is focused north of San Antonio in an Ashe juniper dominated cavernous karst landscape. The goal of this research is to identify and quantify all aspects of the water balance equation under forested and unforested conditions. Using a large scale rainfall simulator allows recreation of rainfall events where amount, rate, and duration of water applied are known. The cave footprint is instrumented with throughfall collectors and rain gauges to quantify throughfall and aid in estimation of interception. Four trees within the plot are instrumented with stemflow collectors and transpiration measurement devices that record data that are later scaled up to account for the entire plot. Drip collectors are constructed inside the cave from PVC pipe and clear polyurethane plastic sheeting that direct recharge to electronic tipping buckets that record onto a datalogger. Continuous automatic recording of cave recharge allows us to see the effects of simulated and natural rainfall events on recharge rates, volumes, and durations. Preliminary analysis of natural and simulated data has shown us that the cave does receive significant quantities of recharge. Thus far, we have concluded that the cave is more sensitive to natural rainfalls than re-created rainfalls, suggesting that the catchment area of the cave is larger than its footprint.
H33B-05 1330h
Exploring Linkages Between Land Use and Hydroecology Using Multivariate Analysis and Process-Based Models
It is well known that land uses influence water quality and ecosystem integrity but the linkage is often poorly understood. The causes of impaired water quality need to be understood to allow for educated land management decisions that will preserve our hydrologic resources and ecosystems for future generations. Frequently, these decisions deal with the mitigation of common stream stressors including low dissolved oxygen levels, increased temperature, and elevated nutrient concentrations. Urban and agricultural land uses are often blamed for harmful effects but it can be difficult to link land use with stream water quality. Statistical methods can infer land use-water quality linkages, but they provide little insight to underlying mechanisms. In contrast, process-based numerical models simulate the necessary hydro/bio/geo/chemical aspects of streams but often require extensive and/or idealized parameterization. This study utilizes a novel approach that combines detailed synoptic water quality data, land use data, multivariate techniques, and process-based numerical models to explore water quality-land use relationships. Principal component analysis identified land use-water quality signatures for 120 source areas ranging from 1 to 47 square km across Michigan. Correlations were established between water quality and land use, as well as with leaf area index, a MODIS data set. These associations were then explored using widely-used water flow and quality models, including QUAL2K and FEMWATER. The models were coupled to account for both surface water and groundwater, since Michigan streams are groundwater-dominated during the summer. This hybrid statistical/modeling approach has the advantage of simultaneously identifying stressor-response relations and mechanistically explaining these links.
H33B-06 1330h
Quantifying Ground-Water Savings Achieved by Salt-Cedar Control Measures: A Demonstration Project
Consumption of ground water by phreatophytes in riparian corridors is thought to be one factor responsible for stream-flow reductions in western Kansas and elsewhere. Extensive phreatophyte-control measures, primarily focusing on invasive species such as salt cedar and Russian olive, are being considered in response to concerns about the impact of phreatophytes on surface-and ground-water resources. At present, there is no generally accepted means of quantifying the ground-water savings that might be gained through these control measures. Micrometeorological methods are often not appropriate for this application because their fetch requirements are too large for narrow riparian corridors. Recently, an approach based on diurnal fluctuations in the water table has been shown to have potential for quantifying ground-water consumption by phreatophytes. A demonstration project is underway to examine the utility of this method for assessing ground-water savings achieved through phreatophyte-control measures. This project is being carried out at a research site in a region of salt-cedar infestation along the Cimarron River in southwestern Kansas. The site has been subdivided into four areas of approximately four hectares each in which different salt-cedar control measures will be applied. Control measures will not be used in one area so that data unaffected by those measures can be obtained throughout the project. Wells equipped with submersible pressure sensors have been installed to monitor water-table responses in the vicinity of the most common phreatophyte communities at the site. A neutron access tube has been emplaced adjacent to each well so that water content in the vadose zone can also be monitored. Changes in water-content profiles will be used to estimate specific yield, a critical parameter in the proposed methodology. A weather station has also been installed on site to monitor meteorological conditions and provide reference ET estimates. Water-level data collected prior to any control activities clearly indicate that the magnitude of the water-table fluctuations is highly dependent on the apparent vitality of the phreatophyte community in the vicinity of each well. After the control measures have been applied, water-level data from the treated areas will be compared to data from the untreated area. That comparison should enable quantification of reductions in ground-water consumption produced by those measures.
H33B-07 1330h
Feedbacks of vegetatoin on summertime climate variability over North America Grasslands
This paper uses the notion of Granger causality to analyze feedbacks of vegetation on summertime climate variability over the North America Grasslands. Results indicate that NDVI anomalies early in the growing season have statistically significant Granger causal relationship with precipitation and temperature later in summer (Jul.-Oct.). The nature of the relationship indicates higher mean values from the preceding months may lead to (or Granger cause) lower rainfall but higher temperatures in July through September. These results suggest that initially enhanced vegetation may dry soil faster than normal, and thereby influence climate variability through the strong soil moisture/precipitation coupling in this region. At the same time, decreasing trends of NDVI over the season also lead decreases in precipitation. Here the decreasing NDVI is a proxy for decreases in underlying soil moisture, which through the same mechanism, will result in decreased precipitation later in the season. Also consistent with this mechanism, a negative causal relationship is identified from temperature to precipitation in summer. However, it is also found that temperature may prompt precipitation at the beginning and the end of the growing season. Such changes of the causal relationship may be related to the seasonal shifts of atmospheric circulation patterns over this region.
H33B-08 1330h
Relative Contribution of Nighttime Transpiration to Daily Total Water Use by Tamarix in a Desert Riparian Woodland
The phreatophytic plant, Tamarix, consumes water from the Pecos River in west Texas, forming a narrow band of dense riparian woodland in an otherwise sparsely vegetated desert environment. Narrow desert riparian zones are largely influenced by dry desert air, creating favorable conditions for nighttime transpiration. In this study, we explore whether nighttime transpiration, especially on nights with warm temperatures and low relative humidity, significantly contributes to daily total water use by Tamarix. Sixteen stems were equipped with thermal dissipation sapflow sensors and monitored during the 2004 growing season. Relative humidity near the canopy did not exceed 70% on 18 of 213 nights and remained below 90% on 77 of 213 nights observed. Sapflow continued throughout most nights, reaching a minimum just before sunrise, as stem water capacitance refilled from the previous day's losses. Antecedent conditions strongly affected the transpirational response to high nighttime evaporative demands mostly likely because of stem water capacitance dynamics during and between rain events. Significant contributions of nighttime transpiration to daily total water use may partially explain how Tamarix is capable of using large amounts of water while maintaining moderate stem conductances. Diel patterns in Pecos River stream flow correspond with evapotranspiration losses from the Tamarix riparian woodland; however, quantitative interpretation must be based on an understanding of both daily and nightly transpiration.
H33B-09 1330h
Integrated approach to ecohydrology of semi-arid sites in areas of complex topography and biome transitions
Vegetation constitutes an essential link in the hydrologic cycle at the land surface - atmosphere interface. Vegetation exerts a predominant control over the partition of rainfall into soil infiltration and evapotranspiration and determines to a great extent the water budget of entire regions. Conversely, water availability has a strong influence on the vegetation dynamics, including growth rates and overall health. In semiarid areas, changes in vegetation composition can follow slight variations in climate-derived moisture availability that, in turn, have a feedback effect on the surface water and energy balance. In this study, we present an integrated approach for studying the ecohydrology of semiarid regions characterized by complex topography and transitions between different vegetation life forms (grasses, shrubs, trees). We first present a set of hypotheses on the interaction between vegetation, landscape conditions, and climate variability in biome transitions zones. These hypotheses will be tested via the combination of numerical modeling and field data collection in the Sevilleta National Wildlife Refuge, central New Mexico. The field site is a first-order drainage basin consisting of two opposing hillslopes that differentially support a shrub-grass ecosystem and a conifer-grass community. Our modeling approaches consist of an ecohydrological module coupled to a 1-D vadose zone model as well as a more complex 3-D ecohydrological framework capable of simulating full vegetation dynamics at the watershed scale. The field instrumentation and model development efforts will be used synergistically to improve our understanding of the ecohydrology of semiarid complex watersheds. We then present an analysis of the ecohydrological model simulations driven by short- term meteorological data (~10 years) as well as by longer term, synthetically-generated climate scenarios. In particular, we will focus on the response of different vegetation communities to the atmospheric forcing and the effects on the hydrologic fluxes, including changes in the water and energy balance. Finally, we point to further potential advances in the coupled simulation of the biosphere, atmosphere, and hydrosphere in semi-arid regions.
H33B-10 1330h
Controls on ecosystem respiration in a semi-arid watershed: seasonality and woody plant encroachment
Shifts in desert vegetation composition, such as woody plant encroachment into natural grasslands, may influence ecosystem processes including CO2 and H2O exchange in unknown ways. During the 2003 and 2004 growing seasons we compared CO2 efflux data from a network of three eddy-covariance towers across a woody plant encroachment gradient within a semiarid watershed. We addressed the following question: how does season influence the primary controls of ecosystem respiration as riparian vegetation structure changes? Towers were located in 3 riparian ecosystems along the San Pedro River in southeastern Arizona: grassland, savanna, and woodland. During the monsoon when soil moisture remained high for substantial periods, variation in temperature accounts for much of the short-term shifts in whole ecosystem CO2 loss. However, during the remainder of the year, when persistent drought was punctuated by ephemeral periods of high soil water availability, temperature played a minor role in controlling the respiration response. In these cases, differences in vegetation structure, their access to groundwater and influence over soil carbon and nitrogen characteristics were more important. Thus, how seasonal climate influences the duration of soil water availability in these southwestern ecosystems hierarchically controls additional drivers, such as temperature, in their influence over ecological processes.
H33B-11 1330h
Comparative Atmospheric Energy Exchanges Over Creosote and Grass Landscapes in Central New Mexico
Creosote vegetation (Larrea tridentata), located in the American Southwest, has been hypothesized to release infrared-absorbing, biogenic hydrocarbons that retards nighttime radiative cooling. Once concentrated in the shallow nocturnal layer, these greenhouse gases could increase the atmosphere's absorption capacity of terrestrial radiation. This increase in greenhouse gases decreases the amount of radiation lost to the overlying atmosphere, thus increasing the radiative energy near the surface. The resulting effect is elevated near-surface temperatures due to a localized greenhouse effect. It is possible that different radiation and energy exchange processes prevail over the creosote vegetation and surrounding grasslands as the grasses are not producers of hydrocarbons. As part of this presentation, we will report on the results from field investigations carried out at the Sevilleta Long Term Ecological Research (LTER) field station in Central New Mexico. Field studies (i) define the thermal characteristics immediately above the creosote vegetation in response to the hydrocarbon greenhouse effect, (ii) determine the thermodynamic attributes of the atmospheric boundary layer over the field site, and (iii) identify the links between synoptic-scale conditions and thermal structure of the lower atmosphere. The data to be included in this presentation come from two twelve-meter micrometeorological flux towers deployed in the middle of creosote and grass vegetated landscapes. Tower sensors allow for the determination of thermodynamic and energy exchange characteristics of the lower boundary layer via meteorological and radiation measurements. Finally, a back trajectory model is employed to determine any large-scale transport of air that may affect the boundary layer conditions experienced at the field site.